1. Field of the Invention
The present invention relates to a tape control apparatus and, more particularly, to such apparatus for controlling the tension of a tape which is transported by capstan and pinch roller between a supply reel and a take-up reel.
2. Description of the Prior Art
In a typical magnetic video recording and/or reproducing apparatus, such as a video tape recorder (VTR), the magnetic tape is normally transported between a pair of reels, so that tape is drawn from a supply reel, then driven by a capstan and pinch roller, and finally wound up on a take-up reel. This basic manner of transportation is the same in so-called open reel machines or in cassette or cartridge machines wherein both the reels are provided in a single, easily manipulatable housing. In all cases it is desirable that the tape be transported with a substantially constant tension, and in general this is achieved by controlling a counter-torque which is applied to the supply reel to create a controlled back-tension in the transported tape.
One factor affecting the maintenance of a constant tape tension is the change in the effective diameter of the supply reel as tape is wound off it. According to the principle of leverage, as this effective diameter decreases, the amount of countertorque which must be applied to the supply reel in order to create a given back-tension on the tape also decreases. Thus, it is desirable to detect the effective diameter of the supply reel, so that the counter-torque which is applied thereto can be modified accordingly. While such effective diameter can be detected by using a mechanical follower in combination with a photosensing device, this type of arrangement may not provide the accuracy which is needed to maintain satisfactorily a constant tape tension.
In a tape control apparatus described in U.S. Pat. No. 3,800,196, a sensing means is provided for developing an electrical signal corresponding to the angular velocity of a supply reel, which is inversely proportional to the effective diameter of the supply reel. The electrical signal is fed to a motor driving circuit, in which a counter-torque generated by the supply reel motor is controlled in response to the frequency of the electrical signal. This countertorque provides a controlled tension in the tape. Thus, irrespective of the change of the effective diameter of the supply reel, the tape is transported with a substantially constant tension.
As described above, therefore, it is possible to effect substantially complete compensation for static tape tension variations which result from the above discussed change in the effective diameter of the supply reel. These static tape tension variations are those occurring during normal, that is to say, steady-state operation. Additionally, however, there are dynamic tape tension variations. These occur because of sudden changes in the rotational speed of the supply reel caused by external disturbances, such as movement, vibration or mechanical shock of the VTR itself. For example, if the supply reel speed suddenly decreases by reason of movement of the VTR, the aforementioned apparatus will operate so as to increase the torque exerted on the supply reel. This means that the rotational speed of the supply reel will be further decreased by the increased counter-torque and the tape tension increased. Thus, it is apparent that the afore-mentioned apparatus cannot satisfactorily compensate for dynamic tape tension variation. Since portable VTRs for professional use are now being produced, it is apparent that there is an immediate requirement for some means to compensate for such dynamic tape tension variations.